We show you how to add remote control to any project, large or small
Posted 08.02.2010 at 10:17 am
8 Comments
Into the Fire
A receiver on the R/C bumper car translates encoded pulse-width modulation signals from the handheld transmitter into commands that enable the connected servos and relays to control the car.
John B. Carnett
You may associate remote control with the urge to jump little R/C cars through walls of fire in your backyard, but that’s just the beginning of what you can do with the technology. Once you’ve mastered the basic concepts, the same parts and techniques used in toys can be used to control machines big and small, practical and absurd.
The basic R/C setup consists of a transmitter, a receiver, and several actuators on the receiving end—either servos (electric motors that rotate to a specified position or speed) or relays (which turn electrical components on and off). Commands from the transmitter about position, speed of rotation, or simply “on” and “off” are sent to the onboard receiver, which decodes them into a signal that can be interpreted by the connected servos and switches.
Although most of the parts of a hobby R/C system can be used in something larger, big machines like a lawn tractor or an excavator often require more torque than a hobby servo can provide. Those machines call for a high-powered super servo. Commercial models exist in the $500 price range, but you can make your own version for $75 or less. In fact, I just did. Some friends and I built gas-powered bumper cars several years ago, but at the insistence of our internal organs, we had had to set them aside. Now, using the DIY super servo for the steering and a hobby servo to control the throttle, the carts have a new life in R/C form. They’re just as much fun as our original bumper cars, with none of the spinal trauma. And I can do flaming-obstacle jumps on a much bigger scale than ever before.
How R/C Works: Phosphor Art
How R/C Works
1. Pulse-width modulation signals indicating servo position or whether the relay switch is on or off are transmitted via radio signal to the receiver.
2. The receiver onboard your R/C project receives the radio signals and translates them into an appropriate set of commands for each servo channel.
3. Signals from the receiver tell the servos and relay switches what to do. Potentiometers use position feedback to ensure that the servos are close to the position requested.
DIY Super Servo: John B. Carnett
Build a Super Servo
Hobby servos are great, but sometimes you need more power. You can make a DIY super servo using the windshield-wiper motor from a car. Here’s how:
1. Get a wiper motor. Try MonsterGuts.com, which sells a 12-volt DC motor for only $16.
2. Attach some means of feedback to measure the position of the motor’s shaft. I used a Futaba s3003 hobby servo ($11; TowerHobbies.com) rewired to act as a potentiometer, but a 5K potentiometer from RadioShack would work equally well.
3. Connect the motor, the feedback potentiometer and the R/C pulse-width modulation signal to an appropriate motor-controller board. The Pololu Jrk 21v3 ($50; pololu.com) is a good choice.
4. Some basic calibration should give you a DIY super servo capable of around 15 pound-feet of torque, enough to control just about any midsize machine. If even that isn’t enough, you can build your servo with a larger gear motor and motor controller.
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They have been doing this same thing on Myth Busters for several of their shows, so this article is a bit late.
It must be pretty awesome to race this karts in a circuit with your friends.
@GabCas It sure is. Especially because you can slam them into each other and flip them over and they won't get destroyed.
Showing the motor controller wiring diagram, and the mechanical linkage between the wiper motor and servo or pot would have been nice. This article leaves the reader feeling like they were taken only half-way there.
psciz Go to pololu.com/catalog/product/1392. Scroll down until you see the video on the 4 ways the Pololu Jrk 21v3 can be used. This should answer a most of your questions, and in addition this board should come with complete wiring diagrams. In the video look for the small gear at the 10:00 position. I believe this drives the feedback pot which in this case is likely a 10 turn pot. The pot in the hobby servo used in the go cart is less than 1 turn.
@psciz Unfortunately, the constraints of space and time for these kinds of articles/projects can leave a lot "as an exercise for the reader" as they say.
I have some pictures that will clarify here:
http://www.flickr.com/photos/vinmarshall/sets/72157624390958773/
You can see the underside of the servo who's potentiometer I'm using for feedback in one of the shots and you can see the mechanical linkage in a few of the other shots.
As @deanosity mentioned, the Pololu Jrk21v3 website and manual make the wiring pretty clear, but post back here if you need any help with it. You're basically just connecting power to the Pololu board and from there to the motor, and then connecting the pot to the Pololu board for feedback. Input to the Pololu board is the PWM signal from the RC receiver.
I chose to repurpose the pot in the servo because it came with it's own easy mounting bracket (the servo body). I used a Futaba s3003 on the first kart and the s3004 (with a ball bearing on the shaft) on the second kart. The rigidity of your linkage to the pot is key to the feedback system - and with the s3003 the shaft of the pot/servo flops around too much causing a bit of overshoot that's hard to chase down. The s3004 worked noticeably better, though was still flexing a fair amount. In reality, once the wheels of the car were on the ground it was a non issue, but I would still probably use a pot with a significantly stronger shaft arrangement were I to do this again.
It also turns out that one of the gears and a shaft inside the wiper motor head were plastic, which took a beating. I'd likely use something like this in the next version:
https://www.surpluscenter.com/item.asp?item=5-1649&catname=electric
You can see more of the kart in action in this video:
http://www.youtube.com/watch?v=d_SlQQsHnK8
Post back here if you have any more questions about how to pull this off.
-vin
I thought about using the windshield wiper servo to control a 4 - 10 hp outboard boat engine. Me and a friend recently purchased a boat... basically a 'fixer upper' and it didnt come with any form of steering. Thought about using this servo in conjunction with an arduino and perhaps an r/c controller like in the article or even a wii nunchuck to control the motor. I really wish I could have seen the wiring between the potentiometer and the wiper motor because I think I could easily adapt this to a boat engine and come on...how sweet would that be lol
hey im going to definitely attempt this project when i get on break from school. I'm wondering however how you got the steering linkage parts, were they on the cart originally? Also how were you able to control the throttle and what did you use for RC communication with the Pololu? Thank you in advanced